A. Field of the Invention
The present invention relates to a finish annealing process for a grain-oriented electrical strip or sheet, and more particularly to a finish annealing process for annealing a grain-oriented electrical strip or sheet in coiled form by means of box annealing using combustion heat obtained from a combustible gas and/or a combustible gas containing liquid fuel.
B. Description of the Prior Art
In the production of a grain-oriented electrical strip or sheet, it is known to subject a hot rolled steel strip adjusted to contain not more than 0.085% C and 2.0-4.0% Si to at least one cold rolling operation combined with heat treatment, to decarburizing annealing, to coating with an annealing separator such as magnesia slurry, to drying, and then to finish annealing in coiled from in a high purity reducing atmosphere at a high temperature for an extended period of time. In the above finish annealing process at high temperature, it is very important for the grain-oriented electrical steel strip to form a coating film having superior properties in connection with electrical insulation, space factor and the like.
It is required for the coating film of the grain-oriented electrical steel strip to have high electric insulation, strong adhesion to the matrix, a high space factor, high heat resistance, uniform properties and uniform appearance.
As is well known, the coating film is formed on the grain-oriented electrical steel strip in the high-temperature finish annealing process as follows: a substance consisting solely or mainly of MgO is suspended in water to form a MgO slurry, the slurry is applied as an annealing separator to the surface of the steel strip which has been subjected to decarburizing annealing, the slurry is dried, and the steel strip is thereafter coiled. Subsequently, the steel strip coil is subjected to the high-temperature finish annealing process.
The annealing separator applied to and dried on the steel strip contains water in the form of free water, H.sub.2 O, and water of crystallization, Mg(OH).sub.2. Expressed in terms of water content, that is in terms of percentage by weight of the total free water and water of crystallization, the water content will usually amount to 10% and more and in extreme cases may exceed 20%. This water is soon evaporated as the temperature rises in the high-temperature finish annealing step.
However, since the steel strip coil is subjected to box annealing, the temperature varies from place to place within the coil and as a result, the rate of water evaporation also differs from place to place. This causes an oxidizing atmosphere to form locally at certain places between overlapped portions of the coil. On the other hand, in the region of the high-temperature finish annealing above the temperature of 950.degree. C., the steel reacts with the magnesia of the annealing separator to gradually form a surface film of the MgO-SiO.sub.2 system, for instance forsterite (Mg.sub.2 SiO.sub.2).
In the gradual formation of forsterite in the above process, particular attention should be paid to preventing pockets of oxidizing atmosphere from being present between overlapped portions of the coil. If an excessively oxidizing atmosphere is present in the spaces between the overlapped portions of the coil, the steel at the surface of the matrix will be oxidized to such an extent that the formation of forsterite (Mg.sub.2 SiO.sub.2) is inhibited. As a result, the film formed during the high temperature finish annealing process will contain much ferrous oxide, which is low in electric insulation.
In order to prevent such defect, an electric furnace equipped with an electric heater has heretofore been used for carrying out the high-temperature finish annealing process. By using an electric furnace, the retained water of the strip coil can be evaporated at the initial stage of the high-temperature finish annealing, stepwise heating including gradual heating and/or low temperature soaking can be easily carried out, and, furthermore, uniform distribution of the temperature within the furnace can be attained so as to minimize the unevenness in temperature.
When the high temperature finish annealing process is conducted in an electric furnace, a surface film with excellent properties can be obtained, but, on the other hand, the energy cost for the electric annealing operation is exorbitant.
In order to overcome the high cost energy problem, consideration might be given to a gas-fired annealing furnace fueled by a combustible gas such as is used for the annealing of low carbon steel strip coils. However, in the case where the high-temperature finish annealing process is carried out in the conventional gas-fired annealing furnace, it is not possible to form a good surface film on the grain-oriented electrical steel. What would be obtained would be a film having inferior magnetic properties degraded by a synergetic effect brought about by the fact that the annealing separator of the grain-oriented electrical steel contains water which evaporates as described earlier and the fact that there is considerable local variation in temperature within the gas-fired furnace.
In addition, in the gas-fired annealing equipment of the prior art, CO.sub.2 gas from the burning of the combustible gas tends to find its way into the hydrogen gas within the inner cover of the annealing furnace. If this CO.sub.2 gas should penetrate to within the inner cover, the magnetic properties of the grain-oriented electrical steel will be degraded. Therefore, the gas-fired annealing furnace has never been used for the finish annealing process of grain-oriented electrical steel in spite of its economical advantage from the viewpoint of energy costs.